The present invention relates to an image forming apparatus such as a copier, printer, facsimile, or similar apparatus, in which an image is formed by an electrophotographic method, and specifically relates to a two-sided color image forming apparatus by which color images are formed on both sides of a transfer material (which is also called a recording sheet).
Conventionally, in two-sided copying, the following method is adopted: one side image formed on an image carrier is transferred onto a transfer sheet (further referred to as a recording sheet), fixed thereon, and the transfer sheet is temporarily accommodated on an intermediate tray; and the recording sheet is fed from the intermediate tray in timed relationship with an image formed again on the image carrier, and the other side image is transferred onto the recording sheet and fixed thereon.
In this two-sided copying apparatus, as described above, recording sheet conveyance such as feeding to the intermediate tray, or two-time passage through the fixing unit, is carried out, therefore, reliability of the recording sheet conveyance is low, resulting in jamming. In contrast to this, a method in which, after toner images are formed on both sides of the recording sheet, fixing is conducted once, is disclosed in Japanese Patent Publication Nos. 37538/1974, 28740/1979, Japanese Patent Publication Open to Public Inspection Nos. 44457/1989, and 214576/1992.
However, in the two-sided color image formation in the above proposals, although the conveyance property of the recording sheet is increased, transfer operations are conducted twice in the rear side image formation from the image carrier to an intermediate transfer body, and from the intermediate transfer body to the recording sheet, as compared to the front side image formation in which the transfer operation is conducted only once from the image carrier to the recording sheet, therefore, the image density of the rear side image is decreased. This situation occurs for the reason that, since the transfer rate is about 90% in transfer, a toner adhering amount is lowered by about 10%. Further, the toner is scattered (a halftone dot is spread and generally .gamma. is increased) due to two-time toner image transferring, and the gradation property is changed. Specifically, in a color image, a color tone problem is newly generated as compared to a monochromatic image. Problems in a color toner image are shown in FIGS. 13(A) and 13(B). As shown in FIG. 13(A), the superimposition sequence of color toner is reversed in the front side and the rear side of the recording sheet, therefore, a color of the toner formed on the uppermost layer is emphasized, or color tone is changed due to toner adhering amount lowering caused by re-transfer, thereby, a problem occurs in which a color image is not finely formed. In order to correct the above problem, as shown in FIG. 13(B), even when a color image is formed in the sequence of black toner K, and Y, M, C color toner, that is, in the order of K, Y, C, M toner image to be superimposed on the rear surface, as opposed to the toner image formation sequence of Y, M, C, K of Y, M, C color toner and black toner K on the front side, a problem is generated in which color tone is different, and a color image is not finely formed.